OK THE KE1,ATIVE IK-TEKSITY OF CHEMICAL FORCE.
TT’e are accustomed in clicmistry to use the words “ chemical attractioii,” o r “affinity,” witliont forming a distinct conception as t o the miinner in which this :iffinit!- may act. T h e different degrees of readiness with which elements combine suggests the question : K h a t is t,he c:Luse of this difference in behavior? Some chemical compo~indsarc less stab!e than others. Even the elements in their pure m o l e c a l ~ rstate are split with greater facility in some c:tscs than others into atonis hefore entering into new compounds. K c :LW ;~ccortoinecl i n this 1:itter case to speak of a resistance whicli tlic niolwulc of a11 (,lenient offers before it enters into combination witli otiicr elements. We note t h a t there is a forcc acting betwceir the atonis of the same element and betweeii those of different eleineiits. I r e call this the clicmical force. Amongst the newer stereoclrcmical coiiccptions there is one, expressed by Ales. S a n m a n n * i i i the following words : ‘ I ‘l’he greatest possible effect of the attractive force between carbon atoms takes place wheii the directions of attraction coin cide with the line which unites thc ceiitres of gravity of t w o such atoms. If the directions of attraction deviate from this line, only t h a t component of the total attraction links the atoms, which lies in the direction of this line.” If this be true, it follows naturally t h a t even two carbon atoms may be held in combination by a greater chemical force than that which holds two other carbon atoms together. T h e reason is to be found in a different gtereochemical arrangement.
* Ber. d. Chem. G e s . , 23, 477,
1890.
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I n applying this idea to the chemical force in general, we find t h a t this force is dependent upon the direction in which it is exerted. Older speculations, notably those of Berthollet, have impressed upon us the idea t h a t the atomic weights have a n influence upon this force, although not in such a general sense as Bertliollet assumed it. We must add to this the principle of valency, which tell us t h a t the atoms of some elements offer one point of attack to t h e chemical force while others offer tw9, three or four points of attack. 'I'hus we consider chemical force to be dependent upon atomic weight, valeiicy and the direction in which this force acts between t h e atoms. From this standpoint we may explbin chemical action with greater c1e:wness than t h e mere use of t h e words affinity or chemical attraction a d m i t of. For example, we find i n the first place, that some clcments are more ready to display chemical force than otliers. A hydrogen molecule is more easily split into atoms than a nitrogeii molecnle. We notice here that for a molecule of an element there is 110 difference of weight to be noticed, since these molectiles iwe supposed to consist of two or more atoms of t h e same weiglit. B u t the difference of valcircy and of the direction of the action of force between such molecules need not be the exclusive cause of such difference in be1i:ivior as is displayed by liydrogeii and nitrogen. T h e univxleiit hydiaogen atom is bound by a chemical forcencting in a straight line t o m o t h e r atom of hydrogen exactly equal t o the first ; this line evidently has the direction of the line which unites the centres of gravity of the two atoms. We purposely neglect here t h e theory of rotation of such a system. Two triatomic nitrogen atoms, forming a molecule, are supposed to represent the figure of a three sided double pyramid, the nitrogel1 atoms occupying two opposite corners.* T h e direction of any of these valencies which are supposed to act along t h e edges of the pyramids, certainly do not coincide with the line uniting the centres of gravity of the two atoms. We surmise, therefore, t h a t although one single valency of the triatomic nitrogen atom is equivalent to one hydrogen valency, t h a t the strength of chemical action within the nitrogen molecule cannot be exactly three times __
* Ber. d. Chem. Ges., 23,11, 1890.
as large as within t h e hydrogen niol(miIc, b u t that it must he smaller to some extent. It has been suggested above that t h e atomic weiglit might, in some way, come into coiisitlcratioii ~ v e i iwlieii niolccules of t h e elemeiits were conceived of. If tlic clieiniciil force is hti1)posed to be the result of the vibixtiiig ether coniing into c o i l t a c t with elemelitilly atoms, i t is (:lt;itr t h a t tliis f o r ( - ( >w i l l rrioi'e getlier tlie lighter atonis th:in t,lie 1ie:ivic.r. tliat c~onsctc~u~~iitiy the heavier atoms arc kopt togtttlicr hy ;ti1 :t tisolu tely stronger pressure. If we a d d thaL this force tintls tlirce 1)oints 01' :ittiick in the nitrogen atom, itntl but one in the hytlrogeii ;ttoni. wv see t h e possibility of t l i r nitrogen inolecale beiiig less rcatly t o display chemicnl forcc towards nioleculcs of o t l i c b i . elcnicsiits t l i ; t t i thc! 11ydrogen 1110 I e(:I1lc. We m:1y cxteiiil tliis esample to t l i u tl(pbreiit iiiono-, di-, tria n d tetrotoniic: elcnieiits. \Ye sltall find, however, tlint osygcii and sulphur, f o r csample, nlthougli both diatuniic i n i ~ r i a i i i compounds, show :L differencc i n regard to r e d i n c w of combining with other elements. The influence of t h e atomic weiglit heiiig perceptible in the above sense. Of course. \vc must allow for the different states of itggregation. Solid sulphnr will be slower to react than gaseous oxygen ; bnt we niay c:oiisider both at t h e same temperatures in gaseous forni, an(l tlius eliminate tliis inflncnco. As an example, we shall find tlwt carboii dioxide is easier to prepare sytithctioally t h ~ me i ~ r b o ndisulpliide, water than Iiydrogen s u 1 phidcb. Conccrniiig the stability of a chemical compound .fbmiietl from atoms of different elements we find sever;tl cases possible. Firstly, such compounds are more or less stable at ordinary conditions, itrid without being acted upon by other chemical elements or compounds. T h i s is illustrated by ainmoiiia gas and nitrogen chloride. The hydrogen atom being fourteen times lighter than t h e nitrogen atom, it will not be able to bend the valency of nitrogen from its original direction, and the three hydrogen atoms may be considered as symmetrically arranged with reference to the nitrogen, T h e chlorine atom being two and a half times heavier than
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nitrogen, the three chlorine atoms representing a weight seven a n d a half times t h a t of t h e nitrogen atom, may influence t h e directions of valency of this latter. A compound is t h u s formed (assuming for brevity's sake t h a t its formula be NCI,) which gives us the conception t h a t chlorine atoms in this case are more nearly in reach of and more likely t o act upon one another to form chlorine molecules, and t h a t nitrogen has a tendency in its valencies to spread out to their original direction, forming a nitrogen molecule with a similarly moving nitrogen atom. T h e whole compound is thus in a state of unstable equilibrium which is easily disturbed with the well known results. I n this particulnr case i t is difficult to decide whether t h e atomic weight of chlorine contributes more to the production of a n explosion than the tendency of t h e curbed nitrogen valencies to spread back and to form an original nitrogen molecule, For the other explosive nitrogen compounds t h e case is a little more complicated, b u t it follows by analogy t h a t the cause of t h e decomposition should not be ascribed to t h e above nientioiied peculiarity of nitrogen alone. I n all the explosives which contain nitro groups (potassium nitrate of gunpowder iiot excluded), it is just as much the tendency of oxygen towards new combinations which causes t h e explosion when oxidable elements are contained in t h e snme compouiid, :LS the above pccnliarity of nitrogen. Why does not nitric acid, when p u t upon copper, explode to form nitrogen, copper oxide and copper nitr:tte ? T h e reason that nitroproducts arc so largely used for explosives is that they are practically manageable, t1i:it they are in other words, iiot tile most explosive bodies. If i t were possible to replace in nitroglycerine the three nitro groups by three CIO, groups, t e should have a more explosive compound. A mixture of potassium chlorate and permanganate can only be pulverized together with t h e utmost care, as the dry salts may blow up during this treatment. There is no nitrogenized explosive substance known which does not contain oxygen as well as nitrogen. There are many chemical compounds containing nitrogen in a semi-molecular state, t h e so called diazo compounds which contain t h e cliromogene group N,, some of which are beautiful a n d toler-
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ably fast dyes and not explosive, while it is easily understood t h a t if such diazo compounds do decompose under the influence of chemical reactioii, the Ilitrogen is set free as such in the molecular state, simply because this molecule of iiitrogeii exists two-thirds preformed in the substance. Also liere we see a marked difference between nitrogen and oxygen. Osygeu i i i a semi-molecular state, so to speak, oftcu shoivs a tendency t o reform its molecule under explosion. Peroxide of lij-drogen and the chlorine or iodine compounds of oxygen illustrate this fact. I t must not Le forgotten that the oxygen moiecule and the sulp h u r molecule, its analogue in this sense, may be considered as containing dipoiar atoms mliicli allow of free rotation around a n axis formed by one of their valencies. This rotation does not exist in the trivalent nitrogen. ‘The esp1:tnation does not seem to be remote, if we admit t h a t this rotation, when suddenly slopped by the beginning of combiliation of oxygen with other elements, causes the production of heat and light. As long as this rotation is part11 existing, however, a s in the peroxides, the tendency to ret u r n to the molecular state is iioticeable, coiisidcriiig t h a t oxidation takes place a t the same time it the opportunity i s offered. We m:~y note liere tlie fact that phosphorus, :tntimony and arsenic, represent elenientary conditions of molecules similar to t h a t of nitrogen. A differcncc i n the I,ehavior of ciiloriiic conipounds of these, when compared with iiitrogen chloride, strongly supports 0111‘ surmise that the atomic: weight is one factor which j i i fluenccs tlie intensity of chcniical attraction betweeii atoms. “lie greater ittomic weights of arsenic, phosphorous o r antimony its compared with nitrogen prevent the influence of the heavy chlorine atom upon the curbing ot the directions of their valencies as we supposed this t o take place in iiitrogen cliloride. It does not: help o u r purpose here to consider the peculiarity of tri- :ind pent- :Itomicity of these elements. I t is sufficient to say, t h a t under certain coiiditions a body m:ty be prevented from offering a11 its points of attack to a cert:tin force. If we take R horseshoe magnet and oiily allow one of its poles to attract a piece of iron while the other remsrins free, wc should not be allowed to speak of a unipolar magnet. l y e may consequently
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settle this question here by assuming the higher number of valencies to be always shown when compounds exist at lower temperatures. If we surround phosphorus pentachloride with a heated mantle we attain a dissociation into chlorine and trichloride, eimilar to heated ammonium chloride which furnishes HC1 a n d NH, gas. I n many of the case8 so far mentioned we recognized reactions, which in a former chemical period were spoken a n d thought of as taking place between t h e atoms as between animate beings, by predisposition rather to combine with this than with t h a t element. T h e mere word affinity, changeable as the sense was in which i t was applied a t diflerent periods, indicates this animation. T h e tendency of modern chemistry is to attempt to reduce these reactions to general principles of physics starting from chemical phenomena and remaining upon the chemical standpoint. We must finally consider another ca*se, which illustrates our endeavor. I t is the one examplified by chlorine, when acting upon bromides or iodides, when either bromine or iodine are liberated. We find here cases suggesting reversed conditions as we considered them in chloride of nitrogen and ammonia when compared. The chlorine atomically lighter than bromine is not able to influence the rotation of its unipolar ally hydrogen (as in hydrochloric acid) or of alkali metal in salts (cs nizrck as the heavier bromine or the still heavier iodine, The compounds of iodine with metals show a remarkable tendency towards decomposition with formation of an iodine molecule, although t h e iodine compounds are still stable when compared to nitrogen chloride. T h e relative intensity of t h e chemical force t h a t acts between atoms to form molecules of elements or of chemical compounds must not be lost sight of in all our chemical views. We havc shown the strong points in favor of a more physical consideration of chemical phenomena from a chemical standpoint and it seems as if most chemical reactions might be satisfactorily explained in this way, even to-day. NEJYPORT, E. I., Aug. 7th, 1890.
